- Suppose that there are lots of programs that would like to use the
functions contained in ``help.c''. For every program, we are going to
have to remember to copy the function headers from ``help.c'' as
function declarations. This is a tedious and error prone procedure.
- Once we have copied over the headers, we will have multiple copies
of the same information. The declaration of the functions appear in
the using file (in our case, ``main.c''), while the implementations
are somewhere else. What might happen as we begin to change the two
programs? Over time, if we're not careful, the declarations in
``main.c'' may become different from the headers in ``help.c''. At
that point, our declarations will be worse than useless-they'll be
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